Proof of igniter and flame sensing device and system

ABSTRACT

A single device for proving both the presence of an igniter and flame in a fuel burner is provided, the device including means such as a ceramic coating for both transmitting actinic radiation emitted by the igniter and generating actinic radiation upon flame excitation. The ceramic or other means is associated with an actinic radiation transmitting element which delivers the radiation to a photoresponsive element whereby a signal is provided which triggers fuel delivery to the burner when the igniter is an igniting mode or thereafter when flame is present. A method of proving igniter and flame is also provided utilizing such device and electronic circuitry adapted to prove its own integrity.

350"96el S1 3 X12 .awoetaal United Stat Mercier 1 1 Sept. 16, 1975 GaryM. Mercier, 3212 Florida Ave. South, St. Louis Park, Minn. 55426 [22]Filed: Dec. 10, 1973 [21] Appl. No.: 423,168

[76] Inventor:

OTHER PUBLICATIONS Holzman, IBM Technical Disclosure Bulletin, Vol. 8,

No. 1, June 1965, pp. 151, 152.

Primary ExaminerWalter Stolwein Attorney, Agent, or FirmWalter N. Kirn,Jr.

[57] ABSTRACT A single device for proving both the presence of anigniter and flame in a fuel burner is provided, the device includingmeans such as a ceramic coating for both transmitting actinic radiationemitted by the igniter and generating actinic radiation upon flameexcitation. The ceramic or other means is associated with an actinicradiation transmitting element which delivers the radiation to aphotoresponsive element whereby a signal is provided which triggers fueldelivery to the burner when the igniter is an igniting mode orthereafter when flame is present. A method of proving igniter and flameis also provided utilizing such device and electronic circuitry adaptedto prove its own integrity.

10 Claims, 8 Drawing Figures PATENTEUSEFIBIQ'IS 3.996221 SHEEIIUF3 j -73HERMo- STAT PROOF OF IGNITER AND FLAME SENSING DEVICE AND SYSTEM Thisinvention relates to the art of detecting the presence of an igniter andflame in a fuel burner system, and especially to a system for sensingand proving both igniter and flame with a single device.

There is a need for a reliable, inexpensive electric ignition and flamedetection system which can be applied to a broad range of applianceburners. This need results from the inadequacies of present ignition andflame sensing configurations.

The constant burning gas pilot is currently the most common means ofigniting the main burner in an appliance. The pilot itself must bemanually lit and it is proven by a thermocouple or mercury expansionflame switch. The main burner flame is not proven.

There are several pilot relight systems commercially available. Thesesystems provide a spark to relight the pilot whenever it is accidentallyextinguished. A few types of electric resistance igniters are alsoavailable. Two such devices are the molybdenum disilicide igniter andthe silicon carbide glow plug. Additionally, there are some main burnerignition and flame detection systems available. The majority of thesesystems provide ignition by means of sparking electrodes. Some systemsspark during the entire burner cycle; others spark for a short durationat the beginning of every cycle. Both types use one of the sparkingprobes or employ a separate high temperature metal probe for sensingflame. These systems use the properties of electrical conduction orrectification and conduction through the flame to prove its existence.Also, there are a limited number of systems which use thermal bi-metaltype switches for sensing burner flame. There are significant drawbackswith each of these ignition and flame sensing systems.

Main burner spark ignition systems suffer from probe degradation,especially in high heat flux burners. The probe spark gap is critical.Oxidation of the probes and condensation on the probes can cause thesesystems to fail by preventing proper sparking or improper biasing of theflame signal. The ceramic insulator can cause problems by allowing waterto condense in its pores, presenting an alternate path for conduction.The ceramic itself can begin to conduct at high temperatures andimproperly bias the system. Radio frequency interference caused by somespark igniter systems is also undesirable.

The constant buming pilot system must be manually lit and usuallymanually relit if it goes out. Pilots are continually on, and areincreasingly being criticized as a waste of energy. Pilots cannotusually be employed with powered burners. Proof of pilot does notnecessarily assure proper and safe main burner operation.

The pilot relighter increases system reliability, but it has thedrawback of all spark systems and does not perform well under conditionsof high humidity. The pilot relighter is too costly for the benefit itprovides, with the possible exception of commercial roof-top gasequipment.

Bi-metal flame sensing switches can only be applied to a limited numberof burner designs. For example, bimetal systems have been used for yearson gas dryers, but the same systems cannot withstand the conditions of ahigher heat flux burner, nor can they respond rapidly enough to meetANSI Standards when applied on products such as residential furnanceswhich retain large quantities of residual heat after flameout.

Electrical resistance igniters eliminate the problems of spark ignition.However, the molybdenum disilicide igniter is very fragile and cannotwork at high flow rates or on high heat flux burners. The siliconcarbide igniter has been successfully applied with a bi-metal sensor asa proof of ignition and flame system in certain gas dryers. But thesystem is especially modified to a specific burner design and has allthe drawbacks previously associated with bi-metal sensing systems.

It is an object of the present invention to provide a sensing devicewhich avoids the above-mentioned drawbacks.

A further object is the provision of a single sensing device whichproves both igniter and flame.

A still further object is to provide a sensing system which is failsafe.

In one embodiment of this invention there is provided an optical sensingdevice adapted for use with a photoresponsive element sensitive toactinic radiation comprising an actinic radiation conducting element,said conducting element being in energizing relationship to saidphotoresponsive means, and cover means in actinic radiation transmissiveassociation with said conducting element, said cover means being heatstable, ambient light opaque, actinic radiation transmissive and actinicradiation emissive upon subjection to flame energy.

In another embodiment, a process for detecting igniter and flame in aburner system is provided comprising 1. arranging the above-definedoptical sensing device in actinic radiation receiving relationship to afuel igniter, said fuel igniter being adapted to emit actinic radiationwhen in the igniting mode,

2. energizing said fuel igniter to said igniting mode wherein said fueligniter emits actinic radiation at a level capable of being sensed bysaid device,

3. receiving said actinic radiation by said cover means associated withsaid actinic radiation conducting element,

4. transmitting said actinic radiation to said photoresponsive elementvia said actinic radiation conducting element whereby saidphotoresponsive element provdes a signal for actuating a valve means,

5. supplying fuel to said burner via said valve means for a timesufficient to cause ignition of said fuel by said fuel igniter in theigniting mode,

6. de-energizing said fuel igniter to a quiescent mode wherein said fueligniter does not emit actinic radiation at a level capable of beingsensed by said device,

7. arranging the device in a location providing direct contact betweensaid device and a flame from said burner,

8. flame heating said cover means to a thermally excited mode whereinsaid cover means emits actinic radiation, and

9. transmitting said actinic radiation to said photoresponsive elementvia said actinic radiation conducting element whereby saidphotoresponsive element provides a signal maintaining said valve meansopen so long as said cover means is in said thermally excited mode.

These and other embodiments of this invention which will be disclosedhereinafter are better understood by reference to the accompanyingdrawings wherein:

FIG. 1 is a side elcvational view of a burner system of this invention;

FIG. 2 is a side elevational view in section of a preferred embodimentof the sensing device of this invention;

FIG. 3 is a side elevational view in section of a portion of anotherembodiment of the sensing device of this invention;

FIG. 4 is a side elevational view in section of another embodiment ofthe sensing device of this invention;

FIG. 5 is a top plan view with block diagrams of the burner system ofFIG. 1 together with a schematic representation of a control systemusable therewith;

FIG. 6 is a schematic diagram of a circuit for use in conjunction withthe sensing device of this invention;

FIG. 7 is a logic diagram for a burner system utilizing the sensingdevice of this invention; and

FIG. 8 is a schematic diagram of another circuit for use in conjunctionwith the sensing device of this invention.

Referring to FIG. 1, sensor 1 is mounted in bracket 3 above and in theflame region of burner 5 and also adjacent to and in actinicradiation-receiving relationship to igniter 7. Burner 5 may be aconventional burner providing a flame capable of thermally exciting thesensor l as hereinafter described. Burner 5 may utilize conventionalfuels. especially gaseous fuels of hydrocarbon or hydrogen composition.Situated atop burner 5 is a grate 9. lgniter 7, also mounted to bracket3, includes lead attaching means 13, ceramic housing and igniter core 17which holds ceramic spiral 18. lgniter 7 emits radiation which activatesa photoresponsive element hereinafter described. lgniter 7 is of theelectrical resistance or hot body type as opposed to the type ofigniters known as spark igniters. Upon sufficient excitation such anigniter emits radiation in the visible and infrared wavelengths as wellas in the thermal region. A preferred igniter is a silicon carbideigniter with a negative temperature coefficient and a power consumptionof approximately 100-350 watts. Such an ig niter is available under thetradename Carborundum.

Sensor I includes a forward portion 19 situated in close proximity toigniter 7 and burner 5 and a rear ward portion 21 situated relativelyremotely from the igniter and burner. Forward portion 19 is so locatedas to be directly within the flame of burner 5.

FIG. 2 depicts sensor 1 in greater detail. Sensor 1 includes anelongated actinic radiation conductive element 25 which is surrounded atforward portion by covering 29. Conductive element 25 includes a firstsegment in the form of an integral, elongated rod, and a second segment33 optically coupled thereto by optical coupling agent 37 to provide apath for actinic conduct actinic radiation, i.e., radiation to which thephotoresponsive element is responsive. For purposes of this invention,actinic radiation is generally in the red visible and infrared region.Additionally, such materials should be adapted to function overprolonged periods of time at the temperatures to which it is subjected,generally at l,200 F. or higher. It should also be noted that the saidconducting element will also generate as well as transmit actinicradiation at the hot end. Preferably, the first segment 30 of conductiveelement 25 is composed of the foregoing materials in an integral rodform. The second segment 33 may likewise be composed of the foregoingmaterials or may be composed of a fiber optic bundle of quartz or othersuitable actinic radiation transmissive material. In the former case,where the first and second segments are of the same composition,conductive element 25 may be of a single unitary construction. Theadvantage of the fiber optic bundle is that it permits angularconfigurations of the second segment 33 so that the path of actinicradiation transmission can be bent as needed to conform to designdemands of system. In particular, this allows the photoresponsiveelement to be located in a relatively thermally remote region to avoiddeleterious thermal effects on the photoresponsive element.

A preferred optical coupling agent 37 is a high temperature, opticalepoxy resin. The epoxy resin is stable at the elevated temperaturesencountered in use, and has generally been found stable at temperaturesin the order of 180 C. or higher. The epoxy resin is very clear withoptical properties similar to quartz and has high tensile andcompressive strength. The coefficients of expansion of the couplingagent and elements to be optically coupled are sufficiently similar thatthe bond remains in tact at the elevated temperatures encountered inoperation. A suitable epoxy resin is available commercially under thetradename Isochem."

Covering 29 is situated in actinic radiationtransmissive relationship toelement 25. In the embodiment of FIG. 2, covering 29 is adherably bondedto the surface of conductive element 25. Covering 29 is actinicradiation transmissive and also capable of generating actinic radiationin situ upon thermal excitation by the flame from burner 5. In addition.covering 29 is heat stable and opaque to ambient light. Covering 29 ispreferably of a ceramic composition. A preferred composition, especiallyfor bonding directly to conductive element 25, is a dry ceramic powderavailable under the tradename Ceramacast 511 mixed with silicon carbide.It was found that 325 mesh silicon carbide improved the bonding,opaqueness, and actinic radiation emissiveness of the ceramic. Theceramic powder/silicon carbide mixture combined with as little water aspossible to attain a thick creamy consistency and the mixture soobtained is then applied by rolling the conductive element, generally inrod form, in the mixture and thereafter curing the coated element atl,000 F. for 3 minutes. The silicon carbide may suitably be present inamounts of from 10% to and preferably 30% to 35% by volume of ceramicpowder.

Covering 29 is quite thin in cross-section, preferably I on the order of0.8 .16 cm. in the case of a bonded coating. In other forms. such as aself-supporting case as depicted in FIG. 3, the thickness is about .16cm. Other suitable materials for covering 29 are metals which arecapable of withstanding the flame temperatures and will exhibit theother properties mentioned above. Stainless steel and inconel are twosuch suitable materials. As noted above, when the covering 29 issubjected to flame from the burner 5, it emits as well as transmitsactinic radiation. This radiation includes radiation in the red visibleand infrared regions. The actinic radiation so emitted is received byactinic radiation conductive element for transmission to thephotoresponsive element.

Covering 41 may, but need'not be, of the same composition as covering29, since actinic radiation transmission and emission are not requiredfor this element of the invention. Covering 41 generally performs thefunction of shielding conductive element 25 from harmful temperaturesand specious light and for the latter purpose is opaque to visiblelight. Suitable materials for covering 41 include ceramics, such assilicon carbide, silicon nitride, alumina or graphite.

FIG. 3 illustrates another form of the sensor 1 of this invention.Conductive element 25 is enclosed in a case 45 which in terms offunctional properties is essentially the same as covering 29 with theexception that case 45 should be structurally self-supporting whereascovering 30 should be bondable to conductive element 25. The gap betweenconductive element 25 and case 45 is generally an air gap sufficientlysmall to allow efficient transmission of actinic radiation from case 45to element 25. A gap of about .03 cm. has been found adequate. In thisembodiment, it is seen that a covering analogous to covering 41 isprovided by case 45.

FIG. 4 depicts still another embodiment of the sensor 1. Conductiveelement 25 includes first segment 30, second segment 33 and couplingagent 37 as in previous embodiments. These elements are enclosed in ametal case 49 having a shoulder portion 53. Case 49 is essentially thesame functionally as case 45 of FIG. 3 and covering 29 of FIG. 2.Shoulder portion 53 of case 49 is bored to accept hardware to attachceramic shield 61 (analogous in function to covering 41 of FIG. 2) tocase 49.

In FIG. 5, the system of FIG. 1 is integrated with a control systemdepicted in block diagram form. In this system, thermostat 65 providesan exciting voltage, on lines 69 and 73, to sensing circuit 77 andconventional timing circuit 81. Timing circuit 81 energizes the hot bodyigniter 7. As igniter 7 self-heats, it emits radiation in visible andinfrared wavelengths as well as in the thermal region. Red and infraredradiation is absorbed by covering 29 at the forward portion 19 ofsensor 1. This radiation is transported through the sensor 1 via thefirst segment 30 of actinic radiation-conducting element 25 to thesecond segment 33, where it is transported to sensing circuit 77.Sensing circuit 77 energizes fuel valve 79, allowing fuel to enterburner 5, where contact with igniter 7 produces flame. The flameimpinges on covering 29 sensor 1. After several seconds covering 29begins to emit actinic radiation which is also transported as above tosensing circuit 77. Timing circuit 81 de-energizes igniter 7 which coolsto a state wherein the actinic radiation emission is quantitativelyinsufiicient to energize fuel valve 79. The sensing circuit 77 maintainspower to fuel valve 79 as long as the flame is maintaining covering 29in the excited state or until thermostat 65 disconnects the system.

Sensor 1 is connected as noted above in energizing relationship to aphotoresponsive element included in sensing circuit 77. Thephotoresponsive element may be of a conventional type and may bephotoconductive or photovoltaic in operation. Suitable photoconductivematerials include cadmium selenide and cadmium sulfide. Suitablephotovoltaic elements include selenium, silicon, and bismuth telluride.The photoresponsive element may be optically coupled to conductingelement 25 by conventional means. Preferably, the second segment 33 ofconductive element 25 is a fiber optic bundle connected at a thermallyremote location to the photoresponsive element by an optical couplersuch as the above-described epoxy resin.

FIG. 6 depicts a sensing circuit for performing the sensing operationsdescribed in connection with FIG. 5. This FIGURE represents the first oftwo embodiments of this sensing circuit. Since the timing sequence canbe performed well by any number of conventional means. this has beenleft out of the detailed circuit schematics and its position relativetothe sensing circuit as shown in FIG. 5. The function of the timingcircuit is merely to de-energize the igniter 7 when the system is tobegin its flame sensing mode.

Thermostat 65 provides 60 Hertz AC voltage at lines 69 and 73. Thesensing circuit provides means for actuating fuel valve 79. The valve isenergized by the control A1 of relay A. Relay A is turned on by thedischarge of capacitor 89 and then held on by PUT 94, resistor 167 andresistor 105. The PUT 94 is triggered into conduction when the anodevoltage exceeds the gate voltage by 0.2 to 0.6 volts. By design of thetriggering elements, which consist of reference photoconductor 101,resistor 113, sensing photoconductor 85, resistors 121, 105 and 109,triggering is accomplished when the sensing photoconductor receivessufficient actinic radiation. The actinic radiation level is sufficientwhen it is indicative that the igniter 7 has reached a state capable ofigniting the fuel to be employed. By requiring photoresponsive element85 to be less than the reference element 101 in resistance for the PUT94 to trigger, several functions are accomplished.

The reference element 101 serves the function of setting the requiredlevel of actinic radiationto be received by photoresponsive element 85,as well as offsetting the effect of changes in the energizing voltageand the effect of temperature on the photoresponsive element 85. Thisisaccomplished by illuminating the reference element 101 with sufficientradiation to es tablish a reference resistance (R The amount ofillumination is selected such that the resistance R will be at a valueslightly greater than the resistance R of the photoresponsive element 85when the igniter 7 is in an igniting mode. The illumination is providedby .lamp 139, the brilliance of which is controlled by resistor 143 andvarible resistor 147. Reference element 101 also compensates forvariations in input voltage since any variations will directlyproportionately affect the brilliance of lamp 139 just as it will affectthe igniter 7. While the effect on the igniter 7 and brilliance of lamp139 will not in practice be precisely the same, the difference will notbe significant in terms of operation of the circuit. Insofar astemperature'effect on the resistance R is concerned, this too will beeffectively compensated for by reference element 101 since the latter isof the same type as photoresponsive element 85 and reference element 101is so physically situated that it will experience substantially the sametemperature environment as photoresponsive element 85.

Resistors 105 and 109 are of equal value. This requires resistors 101and 113 to be greater than resistors 85 and 121. Resistors 113 and 121are also set equal and this in effect requires the sensing element 85 tobe more conductive than the reference element 101 if the voltage at- A(the programmable unijunction transistor anode) is to be greater thanthe voltage established by the photoresponsive elements at B (the PUT 94gate terminal).

In order for fuel valve 79 to be energized, the cirlcuit must storeenough energy in capacitor 89 to turn relay A on when it is dischargedby transistor 99 and PUT 94. These transistors are brought intoconduction by the previously explained triggering elements. Diode 129,resistors 151, 155 and 169 determine the final charge level and the timerequired for the charge to be established. Failure of key triggeringcomponents such as the photoresponsive elements or the transistors willcause premature discharge of capacitor 89 and the fuel valve 79 will notbe energized. lf such failures occur in the previous system cycle orduring the timing period (i.e., when capacitor 89 is charging), thesystem will be protected from unsafe operation.

Capacitor 163, at the base of transistor 99, will charge more quicklythan capacitor 89 to assure that transistor 99 will not discharge onnegative half cycles due to the emitter voltage of transistor 99 beinggreater than the base voltage. Resistors 164 and 165 provide chargingfor capacitor 163.

In operation, after some predetermined time T, capacitor 89 will havereached an energy level sufficient to energize flame relay A. After sometime greater than T, igniter 7 will provide a signal to photoresponsiveelement 85 causing it to become electrically conductive, therebytriggering unijunction 94, which will cause transistor 99 to saturate,thereby discharging capacitor 89 into the solenoid of the flame relay A.The flame relay A energizes and closes its contact A1. Thephotoresponsive element 85 continues conducting and PUT 94 is allowed toconduct current every positive half cycle. this alternating current issufficient to hold flame relay A in the active or energized state byresistors 105 and 167. Diode 168 serves as a holding diode for flamerelay A during negative half cycles.

If either resistor 109 becomes open circuited, or resistors 105 or 167should short, there is not enough current present to turn on the flamerelay A. The circuit is failsafe; any component can fail during a cycle,and the circuit will be safe on the succeeding cycle.

Under normal operation capacitor 89 charges. The igniter 7 reaches fullbrilliance. This brilliance is sufficient for the photoresponsiveelement 85 to be made more conductive than the photoresponsive element101. The PUT 94 triggers into conductionso too does transistor 99trigger into conduction. Capacitor 89 is discharged into relay A andresulting in actuation of valve 79. The igniter 7 continues to glowthereby containing the triggering of PUT 94 on successive positive halfcycles. The timing circuit 81 de-energizes igniter 7. If the burnersystem was functioning, the fuel ignited on contact with igniter 7 andthe resultant flame was allowed to contact the sensing probe 1 duringthe time igniter 7 was left on.

After the burner flame impinges on covering 29 of sensor 1 for severalseconds, the covering 29 will emit actinic radiation in situ which willfurther increase the conductivity of photoresponsive element 85. Hence,the triggering process will continue and fuel valve 79 will remainenergized. When timing circuit 81 deenergizes igniter 7, radiation fromthe flame-activated sensor 1 will be sufficient to maintain triggering.Flame-out will cause the actinic radiation emission of sensor 1 to fallbelow the threshold amount needed to operatively actuate photoresponsiveelement (i.e., the photoconductor will return to a relativelynonconducting state). The triggering process will stop, therebyde-energizing flame relay A and de-energizing fuel valve 79 by openingcontact Al. Consequently, the fuel supply to burner 5 will beinterrupted and the system will not automatically initiate anothercycle.

The operation of the circuit of FIG. 6 can be further understood byreference to the logic diagram of FIG. 7. The symbols employed in thediagram have the following meaning: R35 is the resistance of thephotoresponsive element 85; R is the resistance of the referencephotoresponsive element 101; t is the time at which the system isenergized; and I is the time allowed to test the photoresponsive element85.

At the outset, a 60 Hertz AC voltage is applied by thermostat 79 tolines 69 and 73 of the sensing circuit 77 (Box 600). This occurs at timet Within a time t after t the resistance of photoresponsive element 85undergoes testing. If R is less than the resistance R of the referenceelement 101 (Box 601), this means that element 85 is not workingproperly because it is in a radiation receiving state before the timewhen igniter 7 could reach its igniting mode. This condition isconsidered an unsafe failure condition and the circuit will lock out(Box 602) with the valve 79 off and the igniter 7 on. As a result thesystem will be de-energized (Box 603) and the system is then turned 05(Box 604). This lockout occurs because the PUT 94 is triggered too earlyand continues to be triggered into conduction every positive half cycle.It receives its triggering voltage from the bridge circuit 125 composedof elements 105, 109, 101, 113, 85 and 121. The conductive paths to theflame relay A which include resistors and 25, as well as resistors 151and 159, are not sufficient to turn the flame relay A on.

If the photoresponsive element 85 is properly functioning, theresistance R8 is higher than the resistance R (Box 605). Thus, PUT 94 isnot in a conducting or triggering mode.

If the photoresponsive element 85 has the proper resistance R (Box 605),lockout will occur (Box 602) if either one of two situations prevail:the signal from igniter 7 is insufficient (Box 606), or the signal fromigniter 7 or from any other source, while sufficient, occurs at a time tprior to I (Box 607).

If a sufficient signal is received by the photoresponsive element 85 ata time t after t (Box 608), element 85 will drop in resistance below thevalue R Triggering of the PUT 94 will occur when the voltage at point Ais 0.2 to 0.6 volts greater than the voltage at B. Since resistor 105 isof equal value to resistor 109, and since the input voltage is alwaysmuch greater than 0.6 volts, this differential voltage is established bythe photoresponsive element 85 being only slightly less resistance thanreference photoconductor 101. This triggering point will be voltagedependent as is seen by the equation:

VAB a5 r21)/( ss 121 101 133) where V is the input voltage and V is thevoltage at point A relative to point B. As V increases, the resistance Rat which V equals 0.2 to 0.6 volts (triggering voltage) is higher. Thisis offset by the increased brilliance of lamp 139 at the higher voltage.This causes reference photoconductor 101 to be at a lower resistancebecause the lamp is brighter thereby requiring the same of thephotoresponsive element 85 and cancelling out the effect of increasingvoltage. The same thing holes true in reverse for low voltage. Also,since the igniter will glow more brilliantly at higher voltages, theincreased brilliance of the lamp at these higher voltages makes it agood match for the effects of varying voltage on the igniter.

As previously explained, when the voltage at point A becomes slightlygreater than that at B by the increased conduction of photoresponsiveelement 85, PUT 94 conducts on every positive half cycle. This causescharged capacitor 89 to discharge its energy into the solenoid of relayA, thereby turning relay A on. The continuation of photoresponsiveelement 85 in the low resistance state causes PUT 94 to conduct everypositive cycle thereby holding relay A in the energized state throughthe conductive path of resistor 170 and resistor 105. The flame relay Aturns the gas valve -79 on (Box 609) and holds it on as long as eitherthe igniter or flame signal is received by the photoresponsive element85. After allowing time for the sensor 1 to emit radiation upon flameexcitation, the timer 81 turns the igniter off (Boxes 610, 611). Ifignition did not occur (Box 612) or the flame is of insufficientintensity (Box 613), the circuit will again lockout (Box 614) and returnto the nontriggering mode causing the valve 79 to be de-energized. Theigniter 7 will remain off and the circuit locked out until the entiresystem is recycled by removing power and reapplying it after a fewseconds (Boxes 604, 600). If the flame is sufficient (Box 615), thevalve 79 will remain on (Box 616) until the thermostat 69 de-energizesthe circuit (Box 603) or until flameout occurs (Box 617).

FIG. 8 illustrates another sensing circuit for use in the invention.Certain elements common to the circuits of FIGS. 6 and 8 are designatedby like numerals.

The circuit is energized by applying 60 Hertz AC. voltage at lines 69and 73. The circuit'can be designed to operate at any of theconventional voltages 24, 1 l0, 1 17, 208, 240. Proper sizing of theelements is all that is required. Fuse 21 1 allows resistor 171 anddiode 207 to charge capacitor 175. The R C time constant is chosen toprovide some delay less than the minimum time for the igniter 7 to reachits glowing state. Resistors 183'and 187 are so large relative toresistor 171 that they do not effect the charging level or ratesignificantly. The purpose of resistor 183 and 187 is to provide adischarge path for capacitor 175 so that the circuit will reset in a fewseconds after being de-energized by the total discharge of capacitor175.

' If resistor 171 short circuits it would fail to provide the requiredtiming interval since capacitor 175 will reach its peak value almostinstantaneously. If the resistor 171 short circuits, diode 204 willcause heavy conduction through fuse 211 and it will blow therebydeenergizing the circuit.

Capacitor 175 must fully charge before the proper operational sequenceleading to the opening of fuel valve 79 can take place. If any keytriggering elements 85, such as photoresponsive element referencephotoconductor 101, or SCR 199 have failed in the preceding cycle orfail during the test period, the capacitor 175 will not charge if theseelements have failed in a nonsafe mode. The nonsafe mode is such thattriggering of SCR 199 to the on start would occur without the presenceof actinic radiation. This nonsafe mode causes early triggering therebydischarging capacitor 175 prematurely, and thus, causing circuit lockoutbecause insufficient energy had been stored in the capacitor 175.Capacitor 175 must charge to sufficient voltage to energize flame relay179 by its discharge into the relay.

If the circuit elements are in a safe operating mode, capacitor 175 willbecome fully charged. When igniter 7 reaches full brilliance, thesensing photoconductor will be at the critical value R set by lamp 139and the reference photoconductor'101. This critical value is theresistance expected for the photoresponsive element 85 when it receivesactinic radiation from the igniter 7. Resistor 143 is of the propervalue to set lamp 139 at the proper radiation level to cause thereference photoconductor 101 to have a resistance value R The values ofresistors 215, 219, 235 and 205 are such that the diac 195 will breakinto conduction when the photoresponsive element 85 is at the Rresistance level. The resistors 215, 219, 235 and 205, in combinationwith the diac 195 and the photoresponsive elements 85 and 101, make upthe triggering circuit which can cause SCR 199 to conduct. Thistriggering circuit receives energy on negative half cycles. Thus, it ison negative half cycles when the SCR can be brought into the conductivestate. When the diac 195 breaks into conduction, it allows capacitor 101to discharge through capacitor 205 and across the SCR gate biasingresistor 237. This effects a positive voltage at the gate of SCR 199sufficient to cause SCR 199 to conduct. This conduction dischargescapacitor 175 into relay 179 causing the contact A1 of said relay toclose. The closing of contact A1 energizes fuel valve 79 and the highlyconductive path of diode 203. The conductive path allows capacitor 175to charge each positive cycle. During each positive cycle diode 239 actsas a holding diode for the coil of relay 179 preventing chattering ofsaid relay. During each negative cycle SCR 199 will be triggered intoconduction again causing discharge of capacitor 175 into relay 171. Thischarge, trigger, discharge sequence will continue as long as the sensingphotocell is receiving sufficient actinic radiation.

Reference cell 101 provides much the same voltage and temperaturecompensation as it did in the circuit of FIG. 6. When the voltage to thecircuit of FIG. 8 increases, the voltage across the diac increasesproportionally. However, the increased voltage also causes lamp 139 toglow more brilliantly. This causes the back biasing voltage to diac toincrease. The net efi'ect negates the voltage increase and allows thediac to trigger at the same photoresponsive element 85 resistanceregardless of the voltage variations across the otherwisevoltage-sensitive triggering circuit.

Most important, however, is the temperature compensation afforded by thephotoresponsive reference element 101. The resistance of this elementchanges with temperature during its actinic radiation-receiving mode.This allows the voltage across the diac to be unaffected by thetemperature effects of the sensing photocell resistance.

The safety of flame sensing may be enhanced by the addition of a nematicliquid crystal between the secondary conducting element 33 andphotoresponsive element 85. This junction area could be temperaturecontrolled to give both the photoresponsive element 85 and the liquidcrystal further stability. Also, the liquid crystal can be used as ashutter to generate a pulsating signal to the photoresponsive element85. Such a signal will assure immediate detection of the failure ofphotoresponsive element 85 by the lack of its continually changingresistance, the frequency of which is controlled by the liquid crystalshutter and its associated shutter control circuit.

The flame detection circuitry of this invention proves its own integrityat the beginning of each operating cycle, proves the igniter during theproof of ignition mode and then provides main burner proof of flame.This system provides greater safety in all functional modes than anysystem currently available for application to residential and commercialappliances.

A preferred sensor of the type depicted in FIG. 2 includes a 3 inchlength of 2.4 MM quartz rod as the first segment of the actinicradiation-conductive element. The quartz rod can withstand temperaturesin excess of 1,100C. and is very transparent in the visible and nearinfrared Spectrum, but opaque to the infrared beyond 7.0 microns. Thequartz rod is bonded to a fiber optic bundle with a high temperature,optical epoxy. The epoxy can operate to 180C. It is very clear withoptical properties similar to quartz. It has high tensile andcompressive strength. The fiber optics passes radiation between 0.44microns and 1.3 microns and with an addi tional narrow passband around1.5 microns. The junction at the fiber optic end and all but 1 inch ofquartz are sheathed with a ceramic tube. The exposed quartz is coatedwith a thin coating of a ceramic cement. The cement is not transparentto visible light. The cement can maintain high operating temperaturewithout deterioration. Since the coefficients of expansion of the quartzand cement are close, the cement adheres despite the rapid heating in aflame. Although the ceramic expands more than the quartz, its expansionis very small, and the structure of the ceramic allows this to occurwithout hazard to the bonding properties or to the ceramic itself.

When the probe is inserted in a flame, the thin ceramic coating beginsto emit broadband radiation. The radiation travels down the quartz rodthrough the fiber optic bundle and is received by a photoresponsivedevice.

The system of this invention can withstand the severe environmentof ahigh heat flux powered burner and also reliably prove flame in a muchlower heat flux burner. It is sensitive enough to prove flame for arangetop burner over a reasonable ambient temperature range using thesensor of FIG. 2 or FIG. 4.

The system of this invention enables proof of ignition source as well asproof of flame. It is applicable in any type of burner. It can easily beused on a ceramic burner tile which does not provide the ground returnrequired by many conduction systems. It can be used from 20,0002,000,000BTU/HR/FT of burner surface. The system can get enough signal fromburners as small as a 250 BTU/HR pilot burner. The resistance igniter isnot affected by moisture and is not as position sensitive as a sparkingigniter. The system is low in cost, simple and reliable. The systemeliminates probe deterioration and probe gap problems prevalent in sparkignition systems. Carbon deposits which can prevent ignition and biasflame sensing on other systems do not effect this system.

What is claimed is:

1. A device for sensing the presence of a flame and an electricalresistance igniter in the igniting mode in a fuel burner system incombination with a photoresponsive element sensitive to infraredradiation, said device comprising infrared radiation-conducting meanshaving a portion thereof adapted to be located directly within saidflame, ceramic cover means associated with and'substantially enclosingsaid portion of said infrared radiation-conducting means, said ceramiccover means being heat stable, ,visible light opaque, transparent to thepassage of infrared radiation emitted by said igniter through said covermeans whereby said igniter is sensed by said photoresponsive element andfurther said cover means emitting infrared radiation upon excitation bya flame whereby said flame is sensed by said photoresponsive element.

2. The device of claim 1 wherein said infrared radiation conductingmeans comprises quartz.

3. The device of claim 1 wherein said infrared radiation conductingelement comprises a quartz rod, a fiber optic bundle, and bonding meansfor optically coupling said rod to said bundle.

4. The device of claim 1 wherein said cover means comprises a ceramiccomposition adherably bonded to said infrared radiation conductingmeans.

5. The device of claim 1 wherein said cover means comprises a ceramiccomposition in infrared radiation transmitting relationship to saidconducting means.

6. The device of claim 1 wherein said infrared radiation conductingmeans is in optically coupled relationship to said photoresponsiveelement responsive to said infrared radiation. i

7. The device of claim 6 wherein said photoresponsive element isphotoconductive.

8. The device of claim 6 wherein said photoresponsive element isphotovoltaic.

9. The device of claim 6 wherein a nematic liquid crystal is operativelyinterposed between said conducting means and said photoresponsiveelement.

10. The device of claim 3 wherein said optical bonding means comprisesan epoxy adhesive optically transmissive to said infrared radiation. 7

UNITED STATES PATENT AND TRADEMARK oF IcE CERTIFICATE OF CORRECTIONPATENT NO. 3,906,221

DATED September 16 1975 INVENTOR(S) Gary M. Mercier It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 4, line 29, the word "in" should be --on.

Column 5, line 1, the word "inconel" should be Inconel.

Column 6, lines 23-24, the word "control" should be -contact- Column 7,line 39, the word "this" should be This-.

Signed and Scaled this sixth Day of April1976 [SEAL] A ltes t:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner ofPatentsand Trademarks

1. A device for sensing the presence of a flame and an electrical resistance igniter in the igniting mode in a fuel burner system in combination with a photoresponsive element sensitive to infrared radiation, said device comprising infrared radiation-conducting means having a portion thereof adapted to be located directly within said flame, ceramic cover means associated with and substantially enclosing said portion of said infrared radiation-conducting means, said ceramic cover means being heat stable, visible light opaque, transparent to the passage of infrared radiation emitted by said igniter through said cover means whereby said igniter is sensed by said photoresponsive element and further said cover means emitting infrared radiation upon excitation by a flame whereby said flame is sensed by said photoresponsive element.
 2. The device of claim 1 wherein said infrared radiation conducting means comprises quartz.
 3. The device of claim 1 wherein said infrared radiation conducting element comprises a quartz rod, a fiber optic bundle, and bonding means for optically coupling said rod to said bundle.
 4. The device of claim 1 wherein said cover means comprises a ceramic composition adherably bonded to said infrared radiation conducting means.
 5. The device of claim 1 wherein said cover means comprises a ceramic composition in infrared radiation transmitting relationship to said conducting means.
 6. The device of claim 1 wherein said infrared radiation conducting means is in optically coupled relationship to said photoresponsive element responsive to said infrared radiation.
 7. The device of claim 6 wherein said photoresponsive element is photoconductive.
 8. The device of claim 6 wherein said photoresponsive element is photovoltaic.
 9. The device of claim 6 wherein a nematic liquid crystal is operatively interposed between said conducting means and said photoresponsive element.
 10. The device of claim 3 wherein said optical bonding means comprises an epoxy adhesive optically transmissive to said infrared radiation. 